Computational modelling of crack initiation in a mixing tee subjected to thermal fatigue load

As a part of the European research project THERFAT, thermal fatigue analysis was performed for a mixing tee of an austenitic piping. Due to mixing of the hot (primary) and cold (secondary) water flows, high frequent temperature fluctuations arise which may cause material degradation and cracking. An evaluation of the integrity of the mixing tee is a complex problem which requires the knowledge of thermal-hydraulic boundary condition, material strength and fracture mechanics properties, as well as an appropriate description of elastic-plastic material behaviour under cyclic loading. Data necessary for the structural analysis were provided by different project partners based on their experimental investigations and numerical modelling. This paper concerns with numerical computations of the stress and strain evolution in the mixing zone subjected to thermal fluctuations, as produced by two mass flows with different temperature and flow rates. Thermal calculations were carried out using direct temperature measurements performed by Seichter on a thin-walled piping with a moderate temperature difference T between the hot and cold fluid temperatures. To investigate component behaviour, a fictitious mixing tee with the wall thickness of 10 mm, the outer diameter of 60 mm and the temperature difference up to DT = 250°C was investigated. Since no direct thermal-hydraulic measurements for in-service conditions a reavailable, the experimental results were scaled and used as boundary conditions in the stress analyses. Calculations revealed that under certain combination of the thermal load parameters local plastic deformations can arise, thus leading to crack initiation.